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Poster presentation

Growth habit and developmental phase transitions in plants are acutely responsive
to the quantity and quality of incident light. Information about neighbouring plants
is primarily perceived by changes in the light environment. Arabidopsis is able to
distinguish these changes through the phytochrome (PHYA-E) red light receptors that
perceive changes in the red to far red (R:FR) ratio, and the cryptochrome and phototropin
blue light receptors. Phytochrome, in its biologically active far-red light-absorbing
form (Pfr), mediates its effects by directly binding with the basic helix-loop-helix
(bHLH) phytochrome-interacting factors (PIFs). High R:FR ratios initiate phytochrome
(Pfr)-PIF binding and PIF protein degradation. Low R:FR ratios photo-convert Pfr into
the inactive form Pr allowing the gene expression that leads to a range of adaptations,
including the shade avoidance responses. PIF1 regulates the expression of a number
of genes by directly binding to a G-box element in their promoter. T-DNA insertional
mutagenesis of a number of these genes results in pleiotropic effects associated with
an altered capacity to respond appropriately to environmental cues, including those
signalling changes in soil water status and the ambient light environment. The data
show evidence of a perturbation of light regulated response processes, including germination,
the shade avoidance responses, stomatal conductance, flowering and the circadian clock
perturbation in the PIF-regulated gene mutants. The pif1,3,4,5 quadruple and one of the PIF-regulated gene mutants fail to show shade avoidance
responses in older plants grown under shade inducing conditions. This suggests that
the encoded proteins mediate a parallel pathway in the induction of the SAS. The implications
of our findings for an understanding of how plants appropriately respond to changes
in their environment will be discussed.